The author of ‘French Women Don’t Get Fat’ is back to fix some more gauche food habits.

Mireille Guiliano, the author of the best-selling book “French Women Don’t Get Fat” and a new one called “Meet Paris Oyster,” has a New Year’s resolution for all the young people she sees rushing to work in her Manhattan neighborhood: Ditch the green juice.

In a corner booth at a West Village seafood restaurant, she slathers butter on a baguette as she lists the benefits that her lunch—bread, oysters, white wine—has over those seemingly healthy “green things” from juice bars. Health fads “destroy people’s metabolism,”...